docs: def export(self, nidm_version, export_dir):
          """
          Create prov entities and activities.
          """
          if self.p_unc is None:
              norm_cdf_z = (1.0 + erf(self.equiv_z / sqrt(2.0))) / 2.0
              self.p_unc = 1 - norm_cdf_z
  
          atts = (
              (PROV['type'], self.type),
              (PROV['label'], self.label),
              (PROV['location'], self.coordinate.id))
  
          if self.value is not None:
              atts = atts + (
                  (PROV['value'], self.value),
                  )
  
          if self.p_unc is not None:
              atts = atts + (
                  (NIDM_P_VALUE_UNCORRECTED,
                   Literal(self.p_unc, datatype=XSD_FLOAT)),
                  )
  
          if self.equiv_z is not None:
              atts = atts + (
                  (NIDM_EQUIVALENT_ZSTATISTIC,
                   Literal(self.equiv_z, datatype=XSD_FLOAT)),
                  )
  
          if self.p_fdr is not None:
              atts = atts + (
                  (NIDM_Q_VALUE_FDR,
                   Literal(self.p_fdr, datatype=XSD_FLOAT)),
                  )
  
          if self.p_fwer is not None:
              atts = atts + (
                  (NIDM_P_VALUE_FWER,
                   Literal(self.p_fwer, datatype=XSD_FLOAT)),
                  )
  
          self.add_attributes(atts)